Dockerfile: improve docs (import from 2021-03-05)

Dockerfile

@@ -3,6 +3,7 @@ FROM alpine:latest
 WORKDIR /
 
 # don't add docker from apk because it's quite expensive - we only need the client.
+# we only use the client to run "$ docker exec" inside the containers. we could do this over the REST API though.
 # statically compiled downloads available here: https://download.docker.com/linux/static/stable/x86_64/
 RUN apk add ca-certificates \
 	&& mkdir -p /tmp/docker-install \